The present invention relates to digital remote control devices and, more particularly, to a digital remote control device for performing transmission in a signal format which has an end pulse or pulses indicating an end of the transmission.
A conventional remote control system is shown in FIG. 6. The remote control system includes a transmission circuit 1 for transmitting a transmission code modulated at a certain frequency, a receiving circuit 2 for processing the received signal, a light emission section 3 which consists of a light emitting element, such as a light emitting diode, and receives an output of the transmission circuit 1, and a light receiving section 4 which consists of a light receiving element, such as a photodiode, and receives a light signal (a) from the light emitting section 3 and sends an output to the receiving circuit 2.
The waveforms of pulses representing a 0 and a 1 are shown in FIGS. 7(a) and (b), respectively; i.e., pulse intervals 5 and 6 represent a 0 and a 1 respectively.
The waveform of a conventional transmission code format is shown in FIG. 8. The conventional transmission code 7 consists of a custom code 8 and an instruction code 9 and repeats itself with a repetition period 12.
In operation, the transmission circuit 1 encodes and modulates the information to be transmitted. The light emitting section 3 then transforms it into an optical signal (a) for transmission. The light receiving section 4 then receives the optical signal, and the receiving circuit 2 demodulates and decodes it into an instruction.
The remote control system makes distinction between a bit 0 and a bit 1 by the length of an interval between pulses as shown in FIG. 7, wherein the shorter pulse interval 5 and the longer pulse interval 6 represent a 0 and a 1 respectively.
In the transmission code format of FIG. 8, several bits of 0s and 1s are combined to form a transmission code or word 7 such that the type of an instruction is distinguished according to the data code of the word 7. In order to avoid interference with other remote control systems, a few bits of the transmission code 7 are allocated to the custom code 8 and the remaining bits are allocated to the instruction code 9. For example, if three bits of a 10-bit transmission code are allocated to the custom code 8 and seven bits are allocated to the instruction code 9, eight (=2.sup.3) different systems each having 128 (=2.sup.7) instructions may be made.
When the transmitter has transmitted a transmission code 7 such as shown in FIG. 8, the receiver first decodes the custom code 8 of the transmission code 7 and then decodes the instruction code 9. If the decoded custom code agrees with the code given to the receiver, the receiver executes the instruction.
The uses of a digital remote control system such as shown in FIG. 6 include selecting the channel, adjusting the volume and turning on or off the power switch of a TV receiver, quick feeding and rewinding a tape and setting a start or stop time of a VTR or VCR, and selecting a cooling, warming or dehumidifying function, setting the desired time and temperature and remote turning on or off the switch of an air conditioner. In addition, the digital remote control system may be used in fields of industrial robots and medical equipment.
FIG. 9 shows a use of the digital remote control device for controlling a TV receiver. The transmission circuit 1 and the light emitting section 3 are incorporated into a transmitter 20, while the receiving circuit 2 and the light receiving section 4 are incorporated into the TV receiver 22. 12 channel setting keys 21.sub.1 through 21.sub.12 are mounted on the transmitter 20 for operating the transmission circuit 1. The TV receiver further includes a control section 23 for processing a signal from the receiving circuit 2 to control circuitry of the TV receiver 22 and a channel display section 24 for display a channel selected.
As shown in FIG. 10, when a key 21.sub.1 of the keypad 21 is depressed for a period of time Tm, a series of words 7.sub.1 through 7.sub.7 each containing key data "1" is output continuously for the period of time Tm. The words 7.sub.1 -7.sub.7 are received by the control section 23 via the light emitting section 3 and the light receiving section 4. In response to the input data, the control section 23 displays a number 1 at the channel display section 24 as shown in FIG. 9 and changes the picture selection mode to a No. 1 channel mode to display a picture of channel 1 on the CRT.
A period of time Tw after the key 21.sub.1 is released, a key 21.sub.3 is pressed for a period of time Tn to send a series of words 7.sub.8 through 7.sub.15 each containing key data "3" to the control section 23. Based on the input key data "1" and "3", the control section 23 displays a number 13 at the channel display section 24 while changing the picture selection mode to a No. 13 channel mode.
In the conventional digital remote system, however, if the same transmission code is received more than twice continuously by the receiver because of temporary interruption of the optical signal (a) resulting from the passage of an obstacle through the transmission path or the same transmission code is sent by successively pressing the same key, it is impossible to decide whether the same key is pressed twice or the same key is pressed continuously but the optical signal is interrupted temporarily. This causes the equipment to malfunction.
More specifically, in FIG. 10, if an optical signal is interrupted for a period of time Ts for some reason, the words 7.sub.1 -7.sub.3 and 7.sub.6 -7.sub.7 are received by the control section 24, wherein decision is made that the key 21.sub.1 has been pressed twice for the time periods To and Tr. Consequently, a number 11 is displayed at the channel display section 24 despite the fact that channel 1 has been selected. Such a malfunction may be corrected by pressing other keys, but the key operation becomes complicated. In addition, a malfunction like this is not allowable in the case of controlling a piece of medical equipment.